Polyvinyl alcohol (PVA), a water-soluble synthetic resin, has been used mainly as a raw material for synthetic fibers. Recently, it is used in various fields, for example for production of film materials, emulsifying dispersants, adhesives and binder resins, because of its favorable properties. The PVA resins are produced by polymerization of vinyl acetate monomer and saponification of the polyvinyl acetate obtained, but they contains impurities such as sodium acetate generated in the production process. Thus in the traditional method for production of PVA resin, washing is carried out for removal of impurities from the product after saponification step (see, for example, Patent Documents 1 to 3).
For example in the methods of producing a PVA resin described in Patent Documents 1 and 2, powder or pellets of the PVA resin are washed with water or an organic solvent such as methanol for removal of sodium acetate contained therein. Alternatively in the washing method described in Patent Document 3, a slurry containing a solid-state resin is washed on an inclined conveyor for reduction of the amount of the washing liquid such as methanol.